I have been working on the same thing. First here is a very helpful video that was posted on here before:
He shows a piece of plywood with a cutout in it and a technique on how to use it as a great drill to get the feel of hitting DOWN and THROUGH.
Secondly, I found a fantastic tip that turned my iron striking around almost overnight. Here is the tip, please give it a try. You will be amazed.
Take a golf tee. Put it in the ground at an angle with the sharp pointy end pointing toward the target and the axis of the tee paralled with the target line. Push it into the ground at an angle, then take your iron and practice swinging into the tee with the idea of pushing it right into the ground. If you tumble it out you are not hitting down.
Now start this last drill with a little swing and gradually increase it. When you get to where you can hit it into the ground and take a divot, try it with a ball. Imagine the tee is sticking out of the ball and you are just trying to hit the tee. The ball just happens to be there. You will be astonished at how quickly your ball striking increases....YES...YES....YES
Chessbum....